The following disclosure is based on German Patent Application No. 101 09 031.5 filed on Feb. 24, 2001, which is incorporated into this application by reference.
1. Field of the Invention
This invention relates to a method for reducing the contamination of at least one optical component that is contained inside a beam guidance space and that is held by a frame defining the beam guidance space. The invention further relates to a corresponding beam guidance system for UV light with a frame defining a beam guidance space and holding at least one optical component contained in the beam guidance space.
2. Description of the Related Art
Such optical beam guidance systems are used for example in UV lasers and in lithography irradiation systems for structure-creating UV irradiation of semiconductor wafers. The frame defines the beam guidance space available for the UV light beam trajectory and holds one or more optical components contained therein. In other words, the term frame designates the casing-like and/or frame-like components that have surfaces bordering on the beam guidance space and that limit it and/or that contain one or more optical components.
It is known that in lithography irradiation systems working with UV light, contaminations occur on the surface of the optical components, which may considerably affect their intended optical function, such as the image quality of lenses that are used in projection lens systems of lithography irradiation systems between the position of a structuring mask and the position of a wafer to be irradiated or in the added mask illumination part of such devices.
Traditionally, the occurrence of such contamination effects is attributed to contaminations coming directly from the irradiated substrate or from the gas atmosphere used or to contaminations created at frame surfaces and/or surfaces of the optical components facing the beam guidance space from substances contained in the gas atmosphere after the beam guidance system is manufactured. It is known that such deposits may be removed during the operation using different decontamination procedures (cf. German patent application DE 198 30 438 AI). However, this requires extra effort and possibly interruptions of the irradiation operation.
In the patent specification U.S. Pat. No. 5,602,683, it is proposed to keep the beam guidance space inside the lens frame filled with a gas containing ozone in order to prevent contamination of a lens system with several lens elements arranged in a lens frame enclosing the beam guidance space.
In the patent specification U.S. Pat. No. 5,696,623, oxides of C, S, Sn, Mg, Si, and the like are held responsible for the contamination of the lens system of a UV irradiation system. It is assumed that these oxides are created from components of an atmosphere gas or of degassing components of the lens setup. In order to avoid contamination, it is proposed to introduce a non-oxidizing gas in the space around the lens surface, such as a noble gas, a reducing gas, or an atmosphere gas with most of the oxygen removed.
In the patent specification U.S. Pat. No. 5,685,895, experiments are mentioned from which it was concluded that the sources for contamination of optical elements in lithography irradiation systems are usually found in the environment of the system rather than in the system itself. This is especially the case for ammonia radicals, sulfuric acid and nitric acid radicals, as well as organic silanes. The use of black anodized aluminum as a mounting support or as a shield is blamed as the cause for often observed ammonium sulfate contaminations. The black anodized aluminum contains a diazo coloring agent and undergoes a treatment with sulfuric acid. These substances may be sources of ammonia radicals and sulfuric acid radicals, which compound to ammonium sulfate in a photochemical reaction.
The patent specification U.S. Pat. No. 6,014,263 deals with the prevention of contamination of an optical lens in an oxidizing environment, in particular at high temperatures, such as a lens in a pyrometer for measuring the exhaust temperature of a gas turbine. In the specification, the lens deposits are attributed to the creation of a volatile oxide on the surface of the corresponding lens frame. It has been found that in the high operation temperatures of 650xc2x0 C. and more in the turbine, volatile oxides of chromium and/or molybdenum may be created from frames containing chromium and/or molybdenum, such as stainless steel. These volatile oxides can then reach the lens surface and cause the deposit. As a remedy, it is suggested to apply a protective layer to the surface area of the frame or the mounting support of the lens that is affected by the oxidation. This layer shall prevent the chromium or molybdenum contained in the material of the lens mounting support from coming into contact with the oxidizing atmosphere on the surface. The coating is designed such that under the high operation temperatures, it creates a protective layer of nonvolatile aluminum oxide.
One object of the invention is to provide a method of the previously mentioned type with which the contamination effects in optical components of a beam guidance system working with UV light may be avoided, or at least reduced, in a new and advantageous manner without affecting the operation and/or the beam guidance quality of the beam guidance system. A further object of the invention is to provide a corresponding beam guidance system.
According to one formulation, the invention provides a method for reducing the contamination of at least one optical component that is contained inside a beam guidance space and that is held by a frame defining the beam guidance space, wherein the surfaces of the frame neighboring the beam guidance space are coated at least partially with a degassing barrier layer. As a measure for reducing contamination according to this invention, the surfaces of the frame bordering on the beam guidance space are at least partially coated with a degassing barrier layer. It appears that this measure alone, or in connection with other traditional measures, can considerably reduce the contamination of lenses and other optical components in optical beam guidance systems for UV light.
The inventors have determined that often a considerable contribution to the contamination of optical components in such system is made by degassing from the frame material, which is often a stainless steel material. The degassing barrier layer is chosen by definition so that it blocks such degassing from the frame material completely, or at least partially, i.e., it acts as a barrier for the degassed substances. At the same time, the degassing barrier layer is preferably chosen so that it does not increase reflectivity for the UV light used, i.e., the reflectivity of the surface sections of the frame with the coating is not higher for UV light of the wavelength range used than the reflectivity without the coating. This ensures that the beam guidance quality of the system is not affected by increased reflections of the employed UV light on the degassing barrier layer. The surface coating with the degassing barrier layer can be provided particularly for frames of lenses and other optical components of the system, but may also be used for other casing parts limiting the beam guidance space if need be.
In a preferred embodiment of the invention, the degassing barrier layer has been designed in its reflection behavior such that it does not increase reflectivity for at least the UV wavelengths 157 nm, 193 nm, 248 nm and 365 nm, which are often used in lithography irradiation systems.
Special degassing barrier layers that have good adhesion and a high degassing barrier effect for degassing components typically observed and that reduce reflection for the UV light are a chemically deposited nickel layer, or a silver, gold or tantalum layer. A NiP alloy layer with a minor part of phosphorus may be used, which is chemically deposited in an electrolyte by using sodium hypophosphite as a reducing agent.
Advantageous embodiments of the invention will now be explained in more detail with reference to the drawing.